The present invention relates to a carry noise measuring system for magnetic recording medium and, particularly, to such system for a thin-film magnetic disk in which medium noise, the so-called carry noise, due to a magnetic recording medium in the form of thin-film magnetic disk is discriminated from noise produced in a noise measuring device itself so that carry noise can be measured within a short time.
The magnetic disk is widely used as one of recording media for an external memory device of an electronic computer. Such magnetic disk is checked during its fabrication on noise thereof and on electric recording performance thereof.
The so-called thin-film magnetic disk whose magnetic layer is formed by sputtering or plating, etc., has been used concomitantly to the recent demand of increased memory capacity and memory density of the magnetic disk. It has been known that, in such recording medium, carry noise is produced by the disk itself. Since carry noise affects the memory performance thereof, this noise should be measured.
A carry noise measuring device itself produces noise at a magnetic head and an amplifier of a readout signal system thereof. This noise is inherent to the measuring device itself and referred to as base noise. In measuring carry noise, base noise should be excluded. FIGS. 1a, 1b and 1c show frequency distribution and energy of these noises and a measuring principle for measuring these noises.
The carry noise measurement of the magnetic disk is performed by recording a test or carry signal in the magnetic disk and reproducing it. FIG. 1a shows a total noise spectrum obtained when the test signal is reproduced. The total noise spectrum is composed of a base noise spectrum Nb, a carry noise spectrum Nc overlapped thereon and test signal components overlapped thereon. As shown in FIG. 1a, the test signal includes, in addition to its basic frequency. "1st", high harmonics "2nd", "3rd", etc.
A base noise signal appears in a magnetic head and an amplifier system of the measuring device when there is no input signal to the head. Therefore, its energy can be obtained by measuring effective amplitude values or levels at respective frequencies and integrating or accumulating them.
Carry noise is generated when the test signal is recorded, over a wide frequency range, say, from 0.1 MHz to 10 MHz. A carry noise energy is obtained by integrating effective amplitude values or levels at respective frequencies within the range similarly to the case of base noise signal. In the past, this was done by means of a spectrum analyzer such that the total signal is subtracted by the test signal including the high harmonics at respective frequencies to obtain effective values thereof and an area defined by the frequency spectrum Nc of the carry noise signal over a predetermined frequency range, e.g., from about 0.1 MHz to about 10 MHz, is calculated by integrating the effective values, as energy of the detected carry noise.
In such method of integrating effective values obtained by the spectrum analyzer, it is possible to measure carry noise of a disk with relatively high accuracy. However, it takes long time and, therefore, it is not adequate to use such method in a practical test procedure of disk.